Hackers Nicholas "Drudge" Penree and Tony Hoyle have figured out an easy fix that allows you to add calendar events to your iPod touch. You'll need read/write access to your touch and a full jailbreak but once you do, all you need to do is add the following two lines to the N45AP.plist file inside your Core Services' SpringBoard app.
The plist is found at
Update: Reader Andi notes that you need to put the editableUserData under "capabilities" not under "root" in the property list.
Update 2: If you'd rather use a text editor rather than the property list editor, convert the file to text-based xml. At the Mac command line, you can do this by issuing plutil -convert xml1 filename.plist. To convert back use binary1 instead of xml1.